# How does the O2 saturation test work?

**URL:** <https://boards.straightdope.com/t/how-does-the-o2-saturation-test-work/279254>\
**Category:** Factual Questions\
**Created:** [December 11, 2004, 2:58am UTC](https://boards.straightdope.com/t/how-does-the-o2-saturation-test-work/279254 "2004-12-11T02:58:00Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Sattua](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sattua/32/11906_2.png) [@Sattua](https://boards.straightdope.com/u/Sattua)\
**Post date:** [December 11, 2004, 2:58am UTC](https://boards.straightdope.com/t/how-does-the-o2-saturation-test-work/279254/1 "2004-12-11T02:58:00Z")

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The O2 saturation test involves just clipping a plastic thing–like a clothespin–onto the finger. It is not tight, it does not pierce the skin, but somehow it still works. How?

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**Author:** ![yoyodyne](https://avatars.discourse-cdn.com/v4/letter/y/a9a28c/32.png) [@yoyodyne](https://boards.straightdope.com/u/yoyodyne)\
**Post date:** [December 11, 2004, 3:51am UTC](https://boards.straightdope.com/t/how-does-the-o2-saturation-test-work/279254/2 "2004-12-11T03:51:59Z")

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By shining light through the clipped flesh and measuring the difference in transmission between oxygenated and non-oxygenated blood.

[Some info.](http://www.nda.ox.ac.uk/wfsa/html/u11/u1104_01.htm)

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**Author:** ![St.Urho](https://avatars.discourse-cdn.com/v4/letter/s/8e7dd6/32.png) [@St.Urho](https://boards.straightdope.com/u/St.Urho)\
**Post date:** [December 11, 2004, 11:10pm UTC](https://boards.straightdope.com/t/how-does-the-o2-saturation-test-work/279254/3 "2004-12-11T23:10:10Z")

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Well, it doesn’t actually tell you the percentage of oxygenated blood. What it tells you is the percentage of bound hemoglobin. It can tell this because the bound hemoglobin folds into a different shape.

While typically this doesn’t make a difference, it can in some cases. For example, hemoglobin has a much higher affinity for carbon monoxide than it does for oxygen so in cases of CO poisoning someone could be showing a sat of 100% but still be hypoxic.

Also, it can’t tell you how much hemoglobin the person has, so someone could still be hypoxic with a sat of 100% because their crit is too low.

St. Urho  
Paramedic